(A) Field of the Invention
The present invention relates to an error amplifier and a DC-DC converter and the converting method thereof, particularly to those involving the reduction of jitters by soft start.
(B) Description of the Prior Art
DC-DC converters have been widely applied to the development and design of electronic products. For a portable product, the output voltage of the battery declines when the battery is being discharged. Thus, it is necessary to build a DC-DC converter in the portable product for stabilizing the output voltage to a fixed value. In addition, a DC-DC converter is needed to provide various operating voltages as required by general electronic devices.
As shown in FIG. 1, a conventional DC-DC converter 10 comprises an error amplifier 11, a PWM comparator 12 and a driver 13. The output voltage (Verr) of the error amplifier 11 comes from the following equation: Verr=amplification (Av)×[reference voltage (Vref)−input voltage (IN or n×Vout)]. To prevent the output buffer from producing excessive instantaneous current in start-up, the DC-DC converter 10 will add a slowly ascending waveform to the input of the PWM comparator 12 as a soft start signal. This arrangement will enhance a gradual increase in the output pulse width of the PWM comparator 12, and thereby reduces the instantaneous current from the output-stage transistor and protects the output-stage transistor.
FIGS. 2(a) to 2(c) show an operational timing diagram of the conventional DC-DC converter 10. In FIG. 2(a), a curve 22 represents the input oscillation curve (OSC) of the PWM comparator 12, a curve 21 represents the slowly ascending waveform in soft start, and another curve 23 represents the output of the error amplifier 11. In FIG. 2(b), the output pulse width of the PWM comparator 12 will show a gradually increasing tendency. In FIG. 2(c), both the output curve (Vout) 24 of the DC-DC converter 10 and the input curve (n×Vout) 25 after feedback will ascend gradually so as to reduce instantaneous current.
The disadvantage of the conventional DC-DC converter 10 is that the soft start cannot control the output waveform of the error amplifier 11. Consequently, a jitter occurs since the error amplifier 11 produces a greater transient voltage before returning to the steady state voltage. Such a jitter will not only cause problems during the operation of many electronic devices but also affect the products' reliability.
Therefore, how to reduce or erase jitters in output voltage is extremely crucial since DC-DC converters have become a necessary part of many electronic devices nowadays.